Project Summary Abdominal aortic aneurysm (AAA) is a common vascular disorder associated with inflammation and upregulation of matrix-metalloproteinases (MMP), which cause the local degradation of the extracellular matrix (ECM) culminating in life-threatening rupture. The incidence of AAA is unacceptably high and still accounts for high death rates. There are major knowledge gaps in the mechanisms that contribute to ECM degradation hindering the pipelines of drug-based therapy. Surgery is the only alternative to overcome the burden of patients. The presence of transmural macrophage (M) in the damaged vascular wall is a key hallmark of AAA. Their role in sustaining ECM degradation remains poorly understood. Previous work from our group has identified instrumental molecular signals that foster M activation in AAA. New preliminary data show that M can also be activated by cellular platelet entities, through their non-canonical inflammatory potential. Activated platelets originating from the lungs but not the bone marrow (BM) illustrated enhanced capacities of mediating such effects. The objective of this proposal is to investigate the contribution of immuno-active platelets in regulating M-dependent ECM destruction in AAA. The central hypothesis is that lung-derived platelets are pathological players capable of assisting monocyte recruitment and subsequent activation of M proteolytic phenotype thereby promoting AAA. Our rationale is based on important observations that the depletion of platelets, dampened transmural M content, elastin fragmentation and reduced AAA. Transfusion of lung platelets but not those originating from the BM accelerated the rate of aortic dilation and induced aortic rupture. Single-cell RNA sequencing of AAA and platelet-depleted tissues provided a vista of immunoregulatory networks synchronized by circulating platelets in the vasculature amongst which leucine-rich repeat protein family member was one of the most downregulated in the aorta exposed to platelet depletion and was distinctly enriched in lung platelets and in monocyte-platelets clusters in the blood. Treatment of mice with an inhibitor of platelet activation, reduced monocyte recruitment, M activation and AAA. In our specific aims, will use novel genetic thrombocytopenic mice models combined with BM transplantation of platelet-specific conditional reporter mice as well as conditional deletion of identified target to investigate the contribution of lung and BM platelets in promoting pathological inflammation in AAA. Mechanistic studies will reveal series of pathways and events orchestrated by lung platelets that culminate in injury of the vascular wall. The proposed research is provocative and innovative because we investigate the role of lung platelets in mediating deleterious vascular remodeling, a heretofore- unexamined mechanism in AAA. This contribution is significant since we will test whether specific targeting of leucine-rich protein in platelets from the lungs can protect against AAA and will pave the way for the development of promising preventive therapeutic strategies to curb AAA.